The Syrian regime has carried out indiscriminate and sometimes deliberate airstrikes against civilians that have killed at least 4,300 people since last summer and that amount to war crimes, an international human rights group said Thursday.

Parts of northern Syria — especially areas along the border with Turkey — have in the past months fallen under the control of rebels fighting to topple President Bashar Assad, including several neighborhoods of the northern city of Aleppo, the country's largest urban center.

"The aim of the airstrikes appears to be to terrorize civilians from the air, particularly in the opposition-controlled areas where they would otherwise be fairly safe from any effects of fighting," Ole Solvang of the New York-based group told The Associated Press.

These attacks are "serious violations of international humanitarian law," and people who commit such breaches are "responsible for war crimes," the group said.

Aleppo Media Center AMC via AP

A citizen's image authenticated by The Associated Press shows homes destroyed in what was said to be a government airstrike and shelling in Aleppo, Syria, on Thursday. A rights group has accused Syria of indiscriminate and even deliberate airstrikes against civilians.

Solvang led the HRW team that inspected 52 sites in northern Syria and documented 59 unlawful attacks by the Syrian Air Force. At least 152 people were killed in these attacks, according to an HRW report released Thursday.

In most of the strikes, the regime planes appear to have had no military target in sight, such as armed opposition supporters or rebel headquarters, when they dropped their weapons on civilian areas, the group said.

The 80-page HRW report said that across Syria, more than 4,300 civilians have been killed in attacks by Assad's jets since last July.

The report is the most comprehensive study of Syrian air force operations since last summer, when Assad's forces started to rely heavily on fighter jets to repel rebel advances and reverse their territorial gains.

Officials in Damascus could not immediately be reached for comment on the report.

The opposition now controls large swaths of northern Syria, and last month captured their first provincial capital, the city of Raqqa. Opposition fighters also control whole districts of Aleppo and some key infrastructure in the east, including oil fields and dams on the Euphrates River.

While the rebels have made major gains, they often cannot hold on to the territory because of the regime's superior air power. The continued threat from the air has also stalled efforts to effectively govern rebel-held areas, allowing opposition leaders from the Western-backed alliance only brief excursions into areas under rebel control.

For its part, the Syrian National Coalition has been marred by severe divisions in its ranks since its formation late last year in Qatar, and its leaders are mostly seen as disconnected from the myriad rebel forces fighting inside Syria.

U.S. Secretary of State John Kerry met with opposition leaders in London on Wednesday to discuss ways to step up aid to rebels fighting to topple the regime in Damascus.

The United Nations says that more than 70,000 people have been killed in Syria's 2-year-old conflict. It started with peaceful protests against Assad's rule, inspired by other Arab Spring uprisings, but following a harsh regime crackdown descended into full-blown civil war.